Blue light-emitting diodes (LEDs) are key to modern, energy-saving, environmentally friendly lighting, and have allowed the development of a range of cutting-edge applications, such as power-efficient screens, lasers for Blu-ray recorders, and laser printers. The cost-per-lumen of LEDs has rapidly decreased in the past decade. However, in the endeavor to further improve their light emission efficacy and power output, researchers have encountered an elusive issue of “efficiency droop”; i.e., efficiency decreases with increasing injection current density, which severely hinders the cost-per-lumen reduction. Non-radiative Auger recombination processes and electron overflow have been identified as the dominant origins of efficiency droop. Both Auger recombination and electron leakage are closely related to the carrier distribution within quantum wells (QWs), as the Auger recombination rate is dependent on the n2p (n>>p, which is the case in a blue LED, where n and p are the electron and hole concentrations, respectively), and electron overflow ensues as a result of carrier imbalance and insufficient recombination within the QWs.
When this issue was traced back to the material growth method and device epilayer design, two factors were found to be primarily responsible for the ineffective carrier confinement and insufficient hole concentration in the QWs. One is the strong polarization-induced internal electric field intrinsic to III-nitride semiconductors along the c-direction due to their wurtzite crystal structure. The associated electron-hole wavefunction spatial mismatch within the QWs could result in the decreased radiative recombination rate. The other is poor hole injection into the active region due to the Mg doping challenge, and low hole mobility in p-GaN and the AlGaN electron blocking layer (EBL). Thus, to mitigate the drooping effect, there are two main routes researchers have taken: polarization effect suppression within the QWs; and hole injection enhancement to the active region. Regarding the polarization field engineering, approaches adopted include semi- or n-polar GaN and quantum barriers/wells structure combination manipulations. However, the growth of these semi- or n-polar plane wafers, and the fabrication processes that follow, bring about substantial complication compared to the conventional c-plane LED structures. Regarding the hole injection issue, approaches proposed include doping and thickness engineering in the QWs, insertion of a hole reservoir, and the design of the EBL. Although those approaches have demonstrated alleviated droop effect by percentage, the doping limitation of p-GaN or p-AlGaN still poses a fundamental hurdle to high efficacy achievement. There have been reports on the uses of an n+/p+ GaN homojunction or a p+ GaN/InGaN/n+-GaN polarization junction as the hole tunneling supplier layer. These junctions provided increased carrier injection efficiency and resultant enhanced optical performances. However, this approach complicates the crystalline material growth process, due to the large variation in the indium content, and demands critical doping control for the heavily doped p+ and n+ GaN layers.